The invention relates to compressors. More particularly, the invention relates to compressor capacity control.
In the refrigeration compressor art, one broad class of compressors is reciprocating piston compressors. Each of one or more pistons is reciprocally mounted in an associated cylinder. Reciprocation of the piston displaces a given volume of the cylinder to act as a compressor or pump.
In applications requiring small displacement volume and/or high pressures, it is known to use a stepped piston configuration. Such configurations are often termed “crosshead pistons”. A relatively small cross-sectional area distal portion of the piston is accommodated in an associated portion of the cylinder. A relatively large cross-sectional area proximal portion may be sized for appropriately robust engagement with an appropriately sized connecting rod. An example of such a configuration is shown in US Patent Application Publication 2005/0129543 A1.
In such a crosshead configuration, the proximal region of the cylinder is typically vented to the compressor sump and is not used for compression. However, one prior proposal to use the proximal area for compression is found in U.S. Pat. No. 5,716,197.